PART II.

THE

Order I.-INGENITE ROCKS.

Class I.-GRANITIC ROCKS.

HE typical rock of this class is a crystalline aggregate of quartz, felspars, and micas, nearly universally known as granite or granyte [Celtic gran, Lat. granum, a grain.]* There are, however, rocks called granite in which part of the mica is replaced by other minerals, sometimes to such an extent that the mica is only microscopically visible.

Granite usually is a quaternary or quinary aggregate, containing, along with quartz, one or two felspars and one or two micas; nevertheless, other minerals are often present, more especially pyrite and marcasite.† Some granites are undoubtedly intrusive, while others appear to be only in part intrusive, portions having been formed in situ; that is, having relations as to position with the associated rocks similar to those which now exist. Typical granites weather with a peculiar rough, rugged aspect.

* In Cornwall granite formerly was called growan, from gronen, a grain.

Some of the granites that weather or disintegrate freely appear to have marcasite as an ingredient disseminated in minute grains throughout the mass.

A. Intrusive Granite; Highly Siliceous Granite; Leinster Granite; Oughterard Granite [intruded into the place it now occupies]. An aggregate of quartz and orthoclase with black and white mica; pyrite and marcasite are often constituents; while Haughton seems to believe that most, if not all, intrusive granites also contain albite.

NOTE. Haughton finds the intrusive granites of Cornwall, Devon, Leinster, and Ulster, to contain " quartz, orthoclase, margarodite, and lepidomelane." He has also detected albite.

NITE.

Varieties in Composition.

A. PYRITOUS INTRUSIVE, or HIGHLY SILICEOUS GRAWhen pyrite or marcasite is a component, galenite and chalcopyrite often occur as accessories.

a. Beresyte, Beresite.-Pyritous highly siliceous granite containing gold.

B. ALBITIC GRANITE (Dana).- Containing albite as well as orthoclase.

C. SCHORLACEOUS GRANITE.

With tourmaline (schorl) in addition to the mica.

In the highly siliceous or intrusive granite, as also in all granite, and some of the other ingenite rocks, are veins of segregation, which in the granites generally form two distinct varieties. One kind appears as irregular veins, nests, or patches, that have no deep-seated source, but die out every way, often in very short distances. The other variety makes regular, often dyke-like, veins, from half an inch or less in thickness, to about two or three yards in width. These latter appear to have segregated from the deep-seated fluid or semi-fluid portions of the mass, and to have been forced up into

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the vacancies or the shrinkage fissures caused by the contraction consequent on the consolidation of the cooled portion or crust of the mass. The granite in such veins would necessarily be more siliceous than the granite mass through which they traverse, as basic rocks, although they melt at a lower temperature than the more siliceous, cool sooner and at a higher temperature, losing their heat much more rapidly. Consequently, it appears evident that the portions of a mass of granite which remain longest fluid ought to be more siliceous than the parts that cool first. The dyke-like veins of segregation are undoubtedly more siliceous than the rock which they traverse. Moreover, they are more compact, and finely crystalline. These latter peculiarities may, however, be due to the final rapid cooling when in the veins.

NOTE.-M'Farlane thus writes on the cooling of siliceous rocks"The scoria of iron-furnaces are usually very acid, containing as much as sixty per cent. of silex. They generally fuse at a temperature of 1,450° C. As they flow out of the breast of the furnace, they may be observed to do so very leisurely, to be sluggish and viscid, but nevertheless to continue fluid a long time; and even, in some cases, to flow out of the building in which they have been produced, before solidifying. On the other hand, slags from certain copper-furnaces, or from those used for puddling iron, are more or less basic, containing from thirty to forty-five per cent. of silica. As they flow out they are seen to be very fluid, and to run quickly, but they solidify much more rapidly than iron slag. Yet these basic slags fuse at 1,300° C., or about 150° less than the more acid slags. Those who have been accustomed to observe metallurgical processes will not find it difficult to conceive how a very siliceous slag might continue fluid at a temperature at which a more basic one might become solid."

They often are more or less felsytoid; that is, have the aspect of a felstone or felsyte; but they always contain mica as a constituent, although often

only visible under the lens or microscope. On account of their similitude in aspect to felsyte, also on account of their usual purplish reddish colour, Delesse and others erroneously call them "eurites," although they are quite dissimilar to the original eurytes of Daubuisson. Rose, on the other hand, describes a granite apparently identical with them, and calls it granityte, a name to which there seems to be no objection.

D. GRANITYTE; GRANITITE (Rose).-A compact finely crystalline rock, usually purplish or reddish purple in colour. An aggregate of felspar (orthoclase), quartz, and mica. Black and

white mica seem always to be present, but often in such minute flakes as to be only microscopically visible. Pyrite, and sometimes marcasite, are locally present.

Granityte occurs as veins in granite from mere lines to two or more yards in thickness. It seems to be allied to elvanyte or quartz-porphyry, hereafter described, as sometimes part of the quartz in the granityte seems to have crystallized out prior to the other constituents. Moreover, granityte nearly always weathers evenly like those rocks, and not with the rugged uneven surface, so characteristic of typical granite.

Allied to the intrusive granite are rocks that to the naked eye seem not mineralogically granite, yet petrologically they seem to be part of the intrusive granite, as they always occur associated

*If granityte, as suggested, fills cracks and fissures in the granite, it and elvanyte must necessarily have cooled under somewhat similar conditions. Jukes has described some of the granitytes of the counties of Wicklow and Dublin, Ireland, as elvanytes.

with it, into which they seem to merge, and it into them. They are as follows:

E. FELSITIC GRANITE (King); FELSITE ROCK (Cotta). -"A rock of compact texture, about the hardness of felspar, with dull or smooth conchoidal or fissile fracture; colour yellowish, reddish, grey, greyish, or bluish, weathering white."-Cotta.

To the naked eye this rock does not appear to contain either mica or quartz; however, with a lens, both of these substances can be detected, but usually they appear to be sparingly and partially developed.

F. GREISSEN; QUARTZITIC GRANITE.—Apparently a crystalline granular aggregate of quartz and mica.

Greissen occurs associated with the intrusive granite, and merges into it. Sometimes, indeed, there is felspar developed, but so sparingly and at such wide intervals, that it seems to be more an accessory than an essential of the rock. In some places even the mica seems to be absent, or in such minute particles as only to be detected by a microscopical examination.

Structural Varieties.

G. PORPHYRITIC INTRUSIVE, OR HIGHLY SILICEOUS GRANITE. In which crystals of felspar are largely and conspicuously developed.

H. PEGMATYTE; PEGMATITE [Gr. pegma, a hardened mass]. The second variety of the veins of segregation; very coarsely and irregularly crystallized, the conspicuous constituents, according to Cotta, being "orthoclase, quartz,